Resistant Materials: an aesthetic exploration of antibiotics and antimicrobial resistance

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The disc diffusion test is a commonly used, and important, method for determining antibiotic sensitivity of bacteria in the clinical and research laboratory.  In this test, paper discs impregnated with different antibiotics  are placed onto an agar plate in which bacteria have been spread all over the surface, and the culture is then incubated. If an antibiotic is effective against a particular antibiotic, it will prevent the bacteria from growing, or kill them, so that there will be an area around the disc  where the bacteria have not grown. This is called a zone of inhibition, and its appearance indicates sensitivity of the bacterium to the antibiotic. When a zone of inhibition does not appear, or is very small, then the bacterium is resistant to the antibiotic.

To make this aesthetic interpretation of the disc diffusion assay, I chose to use two naturally pigmented bacteria, Serratia marcescens (Sm), red,  and Chromobacterium violaceum (Cv), purple and to test their sensitivity to three antibiotics, namely Cloxacillin (C), Kanamycin (K) and Trimethoprim (T). Moreover, instead of using the conventional circular discs for the assay and chose to use antibiotic impregnated papers that had been cut into letters.

In the first run of this process,  the sensitivity of Sm and Cv to C and were investigated. After 12 hours incubation it became apparent that CV was resistant C, as it grew right up to the antibiotic impregnated paper, and in fact,  even grew though it.  On the other hand Sm was sensitive to C, as evidenced by the large and clear zone of inhibition (see below).

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Sm left and Cv right. The reversed L shape on the lefthand side of the agar plates is impregnated with K, and the L on the right with C.

 

After 16 hours of incubation, mutant  colonies of Sm that were resistant to C had begun to emerge. A direct visualisation of evolution and the emergence of  antimicrobial resistance (see below).

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A close up photograph of the zone inhibition in Sm caused by C reveals small red C resistant mutants.

 

After 24 hours incubation the C resistant mutants of Sm are clearly visible but neither bacterial strain, Sm or Cv, displays any resistance to K (see below).

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Sm left and Cv right. The reversed L shape on the lefthand side of the agar plates is impregnated with K, and the L on the right with C.

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A close up image of the C resistant mutants of Sm

 

In the second run of this process, the same bacteria were used, but the antibiotic Trimethoprim (T) replaced C and K. In the image below is the first run of this process (top) and run with T (bottom) but before incubation.

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Top, first run of the process. Bottom, second run, left Sm with T, and bottom right Cv with T.

After 12 hours of incubation both Sm and Cv exhibit sensitivity to T, as evidenced by large and clear zones of inhibition(see below).

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Top, first run of the process. Bottom, second run, left Sm with T, and bottom right Cv with T. Note also how the fungal colonies top left are resistant to both C and  K. 

 

In a very unexpected development, after 24 hours of incubation Sm, but not Cv, had become resistant to T. However, Sm in doing so,  had lost its ability to produce its red pigment and thus become white in the process (see below).

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Left Sm with T, and right Cv with T. The T resistant form of Sm is white. There might also be a white T resistant form of Cv emerging at the bottom of the T.

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